The worsening crisis at the Fukushima power station in Japan has led to inevitable comparisons with the 1986 Chernobyl nuclear disaster that killed workers at the plant instantly, caused cancers in the surrounding population and spread radioactive contamination so far that livestock restrictions are still in place at some farms around the UK.

The situation at Fukushima – which the French nuclear agency estimates to be a level six "serious accident" (two up from the one at Three Mile Island in 1979) – is certainly grave and immediately dangerous for those at the site who are fighting to make the crippled reactors and fuel storage ponds safe.

But whatever warnings are now being issued by foreign governments to their citizens in Japan, there are significant differences that set this apart from the catastrophe in Ukraine, even as the US Nuclear Regulatory Commission warned that a pool of spent fuel rods at Fukushima had boiled dry.

At Chernobyl the nuclear reactor exploded after a surge in power that blew the top off the power plant and sent hot fuel – and importantly, its more radioactive fission products – high into the upper atmosphere, where it floated across national borders.

A fire that broke out in the graphite core forced more radioactive material into the air, helping it spread further. The reactor had no containment facility to even slow the release of radiation from the plant.

The Fukushima boiling water reactor is a 40-year-old power plant and it has some glaring design flaws, but the reactors have been switched off for five days, so there is less fresh radioactive material around, and each core is contained within a 20cm-thick steel container, which is then protected by a steel-lined reinforced concrete outer structure. Even in the case of a meltdown, these measures should at least limit the amount of radiation released.

The engineers at the site are working in swift changeover shifts to limit their own exposure to radiation. After a peak in radioactivity during the release of steam from the plant this week, one worker received a radiation dose of 106 millisieverts, according to Japan's Nuclear and Industrial Safety Agency.

That is a dangerous level, but the dose corresponds to less than a 1% risk of fatal cancer in the worker's lifetime, said Richard Wakeford, an expert in radiation epidemiology at the Dalton Nuclear Institute at Manchester University.

But what of the population beyond? The risk from radiation falls off substantially with distance. The authorities have already imposed an exclusion zone of 12 miles around the power station, introduced food bans and dispensed potassium iodide pills to those in the surrounding area. Those pills are to be taken only if a major leak of radiation spills out from the plant and reaches people at high levels.

The radiation is lower at a distance because the particles become dispersed and their radioactivity continues to fall.

Radiation levels have already risen above background levels in Tokyo and the US navy has measured higher levels off the coast, but these are far below the levels that can harm health. Any danger ahead will come from a major and sustained release of radiation.

There are a number of disaster scenarios that the authorities must contend with that could produce a severe radiation leak. The most obvious is that one or more of the reactors goes into meltdown. That can occur if the fuel rods in the core are not cooled enough, and the rods and surrounding cladding melt.

Because this molten material forms a blob it is much harder to cool than when the rods are spaced apart, so it can heat up further and ultimately melt through the bottom of the reactor vessel. If it then causes an explosion and ruptures the secondary containment, it can release radiation into the environment.

The latest nuclear power reactors due to be built in Britain have a built in "core-catcher" that comprises a chute down which the molten core flows until it reaches a reservoir in the ground, said Andrew Sherry, director of the Dalton Institute.

Another doomsday scenario – and one the engineers are battling now – is that one or more of the huge water pools used to store spent fuel boils dry, exposing the fuel rods to the atmosphere where they catch fire. These fuel rods are heavily contaminated with radioactive fission products that could be released directly into the air.

If either of these happens, more radioactive material will be spewed out of the power station without doubt. At the site, the greatest danger would be from short-lived products of the fission reactions that fizzle out quickly in a burst of gamma rays. These burn out so fast that they are not a major problem further afield. "As time goes by, much of the early short-lived radioactivity dies away and you're in a much happier position," said Wakeford.

Any explosion could launch uranium and plutonium fuel – the latter from reactor 3 – into the air. These would remain as particles and would settle near the plant. They are grim environmental contaminants, and could see vast areas ruled out of bounds, but they are only a serious problem to people if they are ingested or inhaled.

For the more distant population, the most serious radioactive substances that would be released are caesium-137 and iodine-131. These are extremely volatile, so can be carried a long way. But dangerous doses are not likely to travel far on the wind. "Unless you're right next to the plant, the vast amount of the dose would be from what you eat and drink," said Neil Crout, who models environmental contaminants at the University of Nottingham.

The danger comes when radioactive iodine and caesium rain down on the ground, on pastureland, for example, and livestock eat it. Cows concentrate radioactive iodine in their milk. Radioactive caesium accumulates in muscles, and in the past has built up in grazing sheep.

The threat to humans then comes from drinking milk and eating contaminated meat. Both can raise the risk of cancer – iodine especially by being absorbed into children's thyroid glands. The iodine pills work by flooding the thyroid with stable iodine so the gland cannot absorb the radioactive form.

"The principal concern the authorities are worrying about, and it is why they have evacuated the area, why they are banning food stuffs, and why they are issuing stable iodine tablets, is that if there is a serious release, you have radioactive iodine. We know from Chernobyl that you've got to limit the dose to the thyroid glands of young children," said Wakeford. A recent report from the UN's scientific committee on the effects of atomic radiation found that a rise in thyroid cancer was the only substantial medical legacy of Chernobyl in the general population.

"What happened at Chernobyl, which was a much more serious accident than this, was that the local Soviet authorities were in denial, they didn't get people out of the area, they didn't evacuate quickly enough, and they allowed children to continue to drink heavily contaminated milk, and as a consequence, many children received high doses of radiation, a sievert and greater, to the thyroid and we've seen thousands of thyroid cancers as a consequence," Wakeford said.

"In 1957 radioiodine was released in the Windscale fire in Cumbria . They monitored it and tipped the milk away. If they had done that at Chernobyl they could have prevented much of the problem."

• This article was amended on 17 March 2011 to correct a misspelling of Neil Crout's name.